The present invention relates generally to the welds between that connect some components of a nuclear reactor pressure vessel; and more particularly to an apparatus and system for replacing the welds between those components.
A non-limiting example of a nuclear reactor, a conventional boiling water reactor (BWR) is shown in FIGS. 1-3. A typical BWR includes: a reactor pressure vessel (RPV) 10, a core shroud 30 disposed within the RPV 10, and a nuclear fuel core 35. The core shroud 30 is a cylinder that surrounds the nuclear fuel core 35, which includes a plurality of fuel bundle assemblies 40 disposed within the core shroud 30. A top guide 45 and a core plate 50 supports each of the fuel bundle assemblies 40.
An annular region between the core shroud 30 and the RPV 10 is considered the downcomer annulus 25. Coolant water flows through the downcomer annulus 25 and into the core lower plenum 55. Feedwater enters the RPV 10 via a feedwater inlet 15 and is distributed circumferentially within the RPV 10 by a feedwater sparger 20, which is adjacent a core spray line 105. Then, the water in the core lower plenum 55 flows upward through the nuclear fuel core 35. In particular, water enters the fuel bundle assemblies 40, wherein a boiling boundary layer is established. A mixture of water and steam exits the nuclear fuel core 35 and enters the core upper plenum 60 under the shroud head 65. The steam-water mixture then flows through standpipes 70 on top of the shroud head 65 and enters the steam separators 75, which separate water from steam. The separated water is recirculated to the downcomer annulus 25 and the steam flows exits the RPV 10 via a nozzle 110 for use in generating electricity and/or in another process.
As illustrated in FIG. 1, a conventional jet pump assembly 85 comprises a pair of inlet mixers 95. Each inlet mixer 95 has an elbow welded thereto, which receives pressurized driving water from a recirculation pump (not illustrated) via an inlet riser 100. Some inlet mixers 95 comprise a set of five nozzles circumferentially distributed at equal angles about an axis of the inlet mixer 95. Here, each nozzle is tapered radially and inwardly at the nozzle outlet. This convergent nozzle energizes the jet pump assembly 85. A secondary inlet opening (not illustrated) is located radially outside of the nozzle exit. Therefore, as jets of water exit the nozzles, water from the downcomer annulus 25 is drawn into the inlet mixer 95 via the secondary inlet opening, where mixing with water from the recirculation pump occurs.
The BWR also includes a coolant recirculation system, which provides the forced convection flow through the nuclear fuel core 35 necessary to attain the required power density. A portion of the water is drawn from the lower end of the downcomer annulus 25 via a recirculation water outlet 80 and forced by the recirculation pump into a plurality of jet pump assemblies 85 via recirculation water inlets 90. The jet pump assemblies 85 are typically circumferentially distributed around the core shroud 30 and provide the required reactor core flow. A typical BWR has between sixteen to twenty-four inlet mixers 95.
Typically, each jet pump assembly 85 includes at least the following. A transition piece 120, a riser pipe 130 extending downwardly from the transition piece 120 to an riser elbow 135. The riser elbow 135 connects the riser pipe 130 to a recirculation inlet 90 along a wall of the RPV 10. A pair of inlet mixers 95 extends downwardly from the transition piece 120 to a pair of diffusers 115 mounted over holes in a pump deck 125. The pump deck 125 connects a bottom portion of the core shroud 30 with the RPV 10. The riser pipe 130 is typically tubular and is oriented vertically within the downcomer annulus 25, in parallel relation to the wall of the core shroud 30. The riser elbow 135 is typically tubular and bends outwardly toward the recirculation inlet 90. The transition piece 120 extends in opposite lateral directions at the top of the riser pipe 130 to connect with the inlet mixers 95 on opposite sides of the riser pipe 130. The inlet mixers 95 are oriented vertically in the downcomer annulus 25 in parallel relation to the riser pipe 130. Restrainer brackets 140, located between the inlet mixers 95 and the riser pipe 130, provide lateral support for the inlet mixers 95.
Typically, the riser pipe 130 is supported and stabilized within the RPV 10 by a riser brace 143 (illustrated, for example, in FIG. 2) attached to the riser pipe 130 and to an attachment wall 149, which is typically a wall of the RPV 10. Commonly, the riser brace 143 is attached to the riser pipe 130 and to the attachment wall 149 by welding. The riser brace 143 ordinarily comprises a yoke 145 and side members 147 extending respectively from opposite ends of the yoke 145 in a spaced parallel relation. Typically, the yoke 145 has an inwardly curved surface between the side members 147, which is complementary to the outer curvature of the exterior surface of the riser pipe 130.
The riser brace 143 is disposed in the downcomer annulus 25 with the riser pipe 130 disposed between the side members 147. The riser brace 143 is normally attached to the riser pipe 130 via a weld between the inwardly curved surface and the exterior surface of the riser pipe 130. Here, the side members 147 generally transverse to the riser pipe 130 and extend from the yoke 145 and respective ends of the side members 147 attach to the attachment wall 149. The ends of the side members 147 are normally welded to the attachment wall 149. Alternatively, the ends of the side members 147 may be welded to an intermediary structure, such as, but not limiting of, braces, blocks or pads, with the intermediary structure being in turn welded to the attachment wall 149. Typically, each side member 147 comprises an upper leg and a lower leg disposed beneath the upper leg in spaced parallel relation therewith. The riser brace 143 generally provides lateral and radial support to the riser pipe 130. In addition, the riser brace 143 is designed to accommodate the differential thermal expansion resulting from RPV 10 operation, and to accommodate for flow-induced vibrations associated with the reactor water circulation system.
Intergranular stress corrosion cracking (IGSCC) resulting from corrosion, radiation and/or stress may occur in the welds between the riser braces 143 and the riser pipes 130 of jet pump assemblies 85 of an RPV 10. Cracks initiated by IGSCC or other causes in the welds between the riser braces 143 and the riser pipes 130 may grow to critical sizes for mechanical fatigue resulting from the vane passing frequencies of the recirculation pumps exceeding the excitation frequency of the riser braces 143. To avoid resonance, the natural frequency of the riser brace 143 should not be nearly equal to the vane passing frequency of the recirculation pumps (at any pump speed). If the vane passing frequency of the recirculation pumps equals or exceeds the natural frequency of the riser brace 143, then the riser brace 143 may potentially enter resonance; possibly to the detriment of the jet pump assembly 85.
A clamp apparatus for mechanically reinforcing the weld between a riser pipe and a riser brace is disclosed in U.S. Pat. No. 7,185,798 B2 to Butler. Here, the clamp apparatus augments the welded connection between the riser brace and the riser pipe. A clamp apparatus for stiffening a riser brace of a jet pump assembly 85 is disclosed in U.S. Pat. No. 6,647,083 B1 to Jensen. Here, the clamp apparatus is applied to the side members of the riser brace to shorten portions of the side members subject to vibration. The clamp apparatus does not attach to the riser pipe and does not augment the welded connection between the riser brace and the riser pipe.
Various clamps used in jet pump assemblies of boiling water reactors are represented by U.S. Pat. Nos. 6,463,114 B1 to Wivagg, 6,490,331 B2 to Erbes, 6,450,774 B1 to Erbes et al, 6,086,120 and 6,053,652 to Deaver et al, and 6,108,391 to Deaver. The Wivagg patent discloses a clamp used in conjunction with a jacking device to restrain the existing jack screws that are welded about the peripheries of the inlet mixers to provide lateral restraint for the inlet mixers within the restrainer brackets. The Erbes ('331) patent relates to a spring clamp for providing a tight fit between an inlet mixer 95 and a restrainer bracket. The Erbes et al ('120) patent discloses a clamp for being installed on a slip joint coupling an inlet mixer to a diffuser. The clamp is used to squeeze the diffuser to impart an oval deformation to the diffuser. The Deaver et al patents ('120 and '652) disclose a clamp apparatus for supporting the lower portion of a riser of a jet pump assembly. The clamp apparatus comprises an elbow clamp, a riser clamp and a bridge coupling the elbow and riser clamps. The riser clamp includes a pair of legs for being disposed on opposite sides of the riser pipe and a hack portion rigidly connecting the legs in fixed relation. The Deaver ('391) patent relates to a clamp having upper and lower clamp elements receiving the outer end of a riser elbow between the upper and lower clamp.
There are a few possible problems with the currently known apparatuses, methods, and systems for dampening the vibration experience by the riser pipe 130. Currently known solutions involve re-welding or integrating with existing welds, which may lead to a repeat failure. These also generally require longer installation time and expose operators to longer period of radioactivity.
Based on the above discussion, operators of nuclear power plants may desire an apparatus and system for reinforcing the connection between a riser pipe 130 and a riser brace 143 of a jet pump assembly 85. The apparatus and system should not require welds between the riser pipe 130 and the riser brace 143. The apparatus and system should reduce a level of vibration experienced by the riser pip 130.